1. Field of the Invention.
This invention is directed to detector circuits, in general, and, more particularly, to circuits which detect specific circuit conditions in relation to devices that prevent and eliminate electro-static charges, for example, by grounding or the like.
2. Prior Art.
There are many types of detector circuits known in the art. These detector circuits are frequently used in conjunction with manufacturing and assembly processes which are performed on or with electrically sensitive devices, such as semiconductor devices (e.g., chips or wafers), circuits including these semiconductor devices, and the like. It is well-known that if, during the manufacturing or assembly process, a technician (or the workstation) develops an undesirable level of static charge, the semiconductor device can be damaged when the potential is discharged. Likewise, if the device is brought close enough to a static voltage field so that a difference in potential can be induced in the device, the device can destroy itself from within. This damage can render the entire device (and its related circuitry) useless and worthless. This damage can be a very costly occurrence. Thus, with only a small "fault", the device becomes defective and all of the "added value" activity, processing, testing at all stages of sub-assembly and final assembly, and field testing is destroyed and the entire product becomes valueless. Additionally, the insidious nature of this type of damage may not be manifested until long after the device, subassembly, and system are in the field for some time, thus significantly increasing the cost of repair.
In the past, several techniques have been utilized to avoid the electro-static charge problem. For example, the conductive or dissipative work surface is "earth" grounded by strap connections or the like on the premise that the charge will be drained off before a problem occurs.
In other cases, flooring (such as a conductive floor mat or conductive tile) is provided at the workstation on the premise that electrical charge will be drained off before a problem occurs.
Additionally, other discharge devices are used in and around the workstation. These devices include electrically conductive straps worn by the worker. For example, straps are worn around a worker's shoes so that the worker becomes connected to grounded, conductive flooring (such as a conductive floor mat or conductive tile). Also, wrist straps of conductive material are used to connect the worker to ground through a suitable resistance (i.e., one megohm). Likewise, conductive materials, bins, cups and shelves are grounded.
All of these techniques are occasionally used in conjunction with each other in extreme cases.
To ensure grounding integrity, circuits have been used to detect continuity at the workstation. In some cases, circuits have been used to detect that the wrist strap is being worn by an operator. In this way, the work is, effectively, connected to ground through an alarm circuit. If the ground connection is broken, an alarm sounds to warn of a fault condition.
While all of the prior art circuits and systems have a certain amount of usefulness, they generally do not fulfill all of the expected safeguards in a completely satisfactory manner. Therefore, additional monitoring techniques and systems are still being sought and developed.